1. Field of the Invention
The present invention relates to a liquid drop detector for use in a device such as a rain sensor.
2. Description of Related Art
Attempts have been made to detect water drops present or adhering upon the front glass (windshield) of a car so as to automatically operate or initiate a wiper. There are already known various methods for achieving such a detection, including an electric detection method, an optical detection method, and so on.
According to the electric detection method, as shown in Japanese Laid-Open Patent No. Hei 8-261974 (1996), there is disposed a transparent electrode of a so-called "comb" shape inside of a glass plate, wherein the dielectric change between the transparent electrodes of the "comb" shape is detected, being caused by adhesion of water drops on the glass plate. The control of closing or opening of a window or of operating a heater is actuated based thereon.
Further, as one example of an optical detection method, in Japanese Laid-Open Patent No. Sho 62-163949 (1987) there is disclosed a construction in which there are provided two light sources, wherein the incident angle of the light from one of these is set to be greater than a critical angle for a total internal reflection to occur upon the detection surface thereof, while the light from the other source is set to be less than the critical angle for the total internal reflection to occur upon the detection surface thereof, thereby enabling discrimination of the presence or absence of water drops upon a glass.
The apparatus of the electric detection method, involves detecting changes such as the electric resistance and the electric capacitance and so on, between the electrodes. However, it is inferior in durability and sensitivity thereof, and in particular, for the windshield of an automobile, it is important that the detected signal corresponds to the sight of a driver or a passenger, therefore, a sensor of the optical method is preferable.
In some situations, even with the optical detection method, it is difficult to detect drops with accuracy since light from an outside source easily enters into a light receiving means in the structure of the conventional detector. In particular when the light emitting means and the light receiving means is attached on the existing windshield later, this problem is very troublesome, as is the operation of adjustments to the optical axis thereof.